Currently, in aircraft, particularly in passenger aircraft, the highest possible level of safety for the people on board, particularly the passengers, is the objective sought.
To ensure the safety of the passengers in case of a crash, rapid evacuation of the passengers is paramount. To do this, aircraft manufacturers seek to improve the safety of the aircraft doors, that is, to protect the doors to avoid any deterioration and deformation of the doors in case of impact in order to ensure that they operate properly. If the door is damaged, particularly during a crash, the passengers onboard the aircraft may not be able to leave the fuselage or may only be able to do so with difficulty. If the access door to the fuselage cannot open, it is necessary to wait for assistance from specialized emergency ground rescue forces to destroy the door in order to enter the passenger compartment of the aircraft and rescue the passengers, which may take a relatively long time. However, it has been shown that an airplane can catch fire quickly, in a matter of minutes, after a crash. On the contrary, if the door is protected and if it can operate and be opened by people onboard the aircraft without necessarily waiting for specialized forces to arrive, then uninjured passengers can leave the aircraft quickly and the rescue teams can enter the aircraft to help the victims as quickly as possible.
In an aircraft, there may be several exit doors, namely:                one or more passenger doors in the front of the aircraft fuselage,        one or more passenger doors in the rear of the aircraft fuselage,        one or more doors in the center of the fuselage, in the passenger cabin, and        one or more baggage compartment access doors under the aircraft fuselage.        
In current aircraft, each door is mounted in a metal structure installed in the aircraft fuselage. This metal structure comprises frames and a set of stiffeners that together form a doorway. The door is thus mounted in a metal door frame, an aluminum frame, for example. Aluminum has the advantage of being a strong metal that is easy to shape and is able to bend under the effect of an impact to absorb the energy from the impact.
An example of a standard doorway is shown in FIG. 1. Doorway 1 is installed in a an aircraft fuselage 2 to support a door that is not shown in FIG. 1 and that is to be mounted in the empty space 3 of doorway 1. Doorway 1 comprises a frame consisting of an inner framework 5, an outer framework 6 and a core placed between inner framework 5 and outer framework 6. Inner framework 5 is intended to receive means for attaching the door. Outer framework 6 creates the connection between doorway 1 and fuselage 2. The role of core 4 is to absorb the energy from an impact. Core 4 connects inner framework 5 and outer framework 6.
Each framework 5 and 6 of the doorway 1 comprises two horizontal crosspieces, 5a, 5b and 6a, 6b respectively, and two vertical jambs, 5c, 5d and 6c, 6d respectively. The unit formed of an upper crosspiece 5a and a lower crosspiece 5b, of a right jamb 5c and a left jamb 5d, assembled together, constitutes a framework 5.
Traditionally, core 4 between the inner framework 5 and the outer framework 6 is produced using a number of stiffeners 4a, 4b, 4c, . . . 4n, called intercostal fittings. These intercostal fitting are independent components and, more specifically, horizontal stiffeners between the jambs of the doorway and vertical stiffeners between the crosspieces of the doorway. These intercostal fittings are attached on either side to inner framework 5 and to the outer framework 6. Currently, the metal frame of a doorway is optimized so that the intercostal fittings bend during an impact. By bending, the intercostal fittings absorb the energy from an impact, thus protecting the door. This type of metal frame thus makes it possible to comply with the safety standards for aircraft doors.
Aircraft manufacturers try to reduce the weight of the aircraft to the maximum extent possible in order to reduce the aircraft's fuel consumption. One of the ways to accomplish this is to select lighter materials.
The current trend in aeronautics is to replace metal components with components made of a composite material. In fact it is well known that composite materials make it possible to reduce the weight of the aircraft and therefore reduce its fuel consumption. However, in the case of a doorway, it is not sufficient to replace aluminum with a composite material. Indeed, replacing the aluminum intercostal fittings with intercostal fittings made of a composite material would present a problem with respect to energy absorption, for composite materials have a low energy absorption capacity. A composite material subjected to high pressure does not bend. It breaks. In the event of a crash, the stress in a doorway such as the one that has just been described, but with intercostal fittings made of a composite material, would be completely transferred to the door with risks of deformation that could prevent its subsequent operation. The energy from the impact would therefore not be absorbed by the doorway, which would result in the deterioration of the door.
Additionally, the standard architecture causes the stiffeners to undergo shearing action near the base, which is the weak point of composite materials.
Furthermore, making intercostal fittings out of a composite material to produce a doorway as described earlier would be relatively costly from a production standpoint. Manufacturing a high number of intercostal fittings and attaching each fitting between the inner and outer frameworks would result in too great an increase in production costs and in production cycles that are too high compared with current demands.